Intraocular lenses are lenses implantable into the eye for the purpose of the change of correction. They can be implanted into various parts of the eye, such as the posterior chamber, anterior chamber, or stroma. Intraocular lenses are made of material of various rigidity. In case of hard materials, such as certain poly(alkylmethacrylate)s, especially polymethylmethacrylate, or dehydrated hydrogels, also called xerogels, the most common manufacturing process is lathe machining with subsequent polishing. Soft intraocular lenses are usually made by casting in appropriate molds. Casting is based on filling the mold with a liquid polymer precursor, such as for instance a mixture of specific monomers, a melted polymer, or a liquid pre-polymer with a cross-linking capability, and on the subsequent conversion of this material into a solid state, known as solidification. In case of soft materials for intraocular lenses, this solidification generally includes cross-linking, i.e. the formation of a three-dimensional covalent network, which stabilizes the shape of the lens. For example, this network can be formed by either the copolymerization of bi-functional monomers with multi-functional cross-linking co-monomers, known as cross-linking agents or cross-linkers, or the additional cross-linking of the liquid polymer.
The transition from a liquid to a solid state is accompanied by a greater or lesser volumetric contraction, which greatly complicates the casting of the product, for which a great shape accuracy is required, such as in case of intraocular lenses.
The casting process can be realized relatively easily using liquid precursors, in whose case cross-linking results in a small volumetric contraction. An example of such liquid precursors are silicon rubbers.
Most contemporary intraocular lenses is based on the cross-linking of acrylic or methacrylic polymers, however, which are generally made using cross-linking copolymerization of a mixture of acrylic and/or methacrylic monomers. Such monomer mixture contains at least one type of a monomer with one polymerizable double bond, representing basic monomer or monomers, and at least one type of a monomer with two or more copolymerizable double bonds, representing the cross-linking co-monomer or co-monomers.
Basic monomers form the main polymer chain in course of polymerization, while the cross-lining co-monomers form covalent bridges between the chains. The result of this copolymerization is the formation of a three-dimensional network, which is non-meltable and insoluble in any solvent. The process described above is often used in case of hydrogels.
If, for example, the basic monomer is 2-hydrozyethylmethacrylate copolymerized with a small amount, usually no more than 2% molar, of ethylendimethacrylate functioning as a cross-linker, it will form a cross-linked poly(hydroxyethylmethacrylate). Such hydrogels are described, for example, in the U.S. Pat. Nos. 2,976,576 and 3,220,960, and are the basis for many contact lenses and various implants, including intraocular lenses.
Ophthalmic lenses are made using various shaping processes, for example polymerization in closed molds. Closed molds are not particularly suitable for cross-linking copolymerization, however, which is followed by a significant volumetric contraction amounting to up to 20% of the volume. If the volume of the closed mold cavity is constant, then the contraction results in the decrease of pressure within the molds, which has many undesirable consequences, particularly the formation of cavities, bubbles, vacuoles, and surface defects. The contraction during solidification is a common problem in the shaping of plastics, which is solved in various ways, for instance gradual addition of additional liquid precursor, by which the loss of pressure due to contraction is compensated, such as during injection molding of thermoplastic resins.
This technique is practically unusable in case of cross-linking copolymerization, however, because the gel point, which is a state at which the three-dimensional network is created and the flow capability of the copolymerized product is halted, is here reached at very low degrees of conversion. A significant contraction occurs at the point where it is not possible to add further liquid precursor.
These difficulties during cross-linking copolymerization in closed molds led to the search for alternative casting technologies. The casting of optical components, such as ophthalmic lenses, requires an extremely good shape definition, excellent surface quality, and material homogeneity, in other words qualities unattainable if the overpressure in the cavity of the closed mold is not maintained. The patent literature shows many proposed solutions to this problem. One of these is the casting while the mold is rotating, which is known under it's original name “spin-casting”, and which was, for example, proposed for the production of hydrogel contact lenses in the U.S. Pat. Nos. 3,660,545, 4,517,138, 4,517,139, 4,517,140 and 4,551,086. This technique is used for the manufacture of contact lenses using an open concave mold, provided with a sharp edge forming the boundary of the form. The mold is filled with a small volume of a monomer mixture, which is significantly smaller than the volume of the concave cavity of the mold. The level of Liquid monomer mixture is, in this case, always deep under the plane of the sharp mold edge. Due to the mold rotation around the vertical axis of symmetry of the mold cavity, the liquid monomer mixture will spread to form a concave shape, approaching a paraboloid. The result is a convex/concave lens, whose central thickness is very low, which means that it is significantly lower than the sagital depth of the mold cavity. This shape is especially appropriate for hydrogel contact lenses. The speed of mold rotation for the manufacture of contact lenses typically lies between 300 and 500 rpm, with the mold cavity diameter in the plane of it's sharp edge typically being between 13 and 17 mm.
Another known use of rotational casting is the formation of parabolic mirrors for telescopes and other instruments requiring precise focusing. In these cases, the goal is the creation of parabolic optical surfaces with a single focus for coaxial rays and without spherical aberration.
The U.S. Pat. No. 3,691,263 describes a different method of rotational casting, where the casting of contact lenses is done without the use of molds. It involves the polymerization of liquid monomers on the surface of a rotating carrier liquid, which is non-miscible with polymerizing liquid monomers, which has a higher density than polymerizable liquid monomers, and which is for example mercury or a concentrated saline solution.
Another modification of this process uses monomer polymerization on the interface of two rotating non-miscible liquids, of which one has higher and the other one has a lower density than the starting monomer and the resulting polymer.
The U.S. Pat. No. 4,806,287 describes a method of lens manufacture using hydrophilic gels. This method is based upon the solidification of a droplet of a monomer under a non-miscible liquid, such as oil, while at least the optical zone of the lens is formed between a mold and an appropriately shaped plunger. One option mentioned is also mold rotation, where the mold rotation does not affect the optical zone of the lens, which is defined by the shape of the plunger, but it affects only the peripheral parts of the molding. For that reason, this lens cannot be considered a rotationally-cast lens.
Rotational casting on liquid surfaces of a high specific gravity can be also used for the casting of precise tubing made of cross-linked polymers, as described by the Czech patent CZ 153760.
So far, rotational casting has not been utilized for the production of intraocular lenses for several reasons. First of all, most intraocular lenses made until now has a relatively small diameter, which is usually equal to no more than 6 mm. It was assumed that centrifugal casting would have only a limited effect on the shape of the lens for such diameters, because the centrifugal force increases with the square of the distance from the rotation axis. Beside that, intraocular lenses are generally biconvex, or less frequently planar-convex lenses, and rotational casting has been up to date used only for lenses which are distinctly convex/concave lenses, such as in the case of contact lenses. Finally, contemporarily manufactured intraocular lenses often have complex shapes of a non-circular footprint, with the optical zone and integral haptics for centering the lens in the eye. Such shapes with non-circular shape are not suitable to rotational casting in open molds. Due to the reasons listed above, rotational casting of intraocular lenses has been until now considered unfeasible by the experts in the field.
Until now, intraocular lenses have been made using casting into open stationary molds, where the meniscus formed by the liquid monomer defined the shape of one of the optical surfaces. These methods are described in the U.S. Pat. Nos. 4,971,732, 4,846,832, and 5,674,283. According to the U.S. Pat. No. 4,971,732 the liquid monomer mixture is metered into a concave cavity provided with a sharp edge forming the boundary of the cavity. The material from which the mold is made is poorly wettable by the monomer mixture. The volume of the liquid monomer mixture dosed into the mold must be equal or preferentially higher than the volume of the mold cavity, so that the level of the liquid monomer mixture will reach up to the sharp edge of the mold. If the amount of the liquid monomer mixture is insufficient is insufficient in this respect, the liquid filling the mold cavity will retract away from the sharp edge due to polymerization contraction; therefore it is impossible to obtain a quality product in this case. Therefore it is beneficial to increase the metered volume of the liquid monomer mixture and thus attain a higher meniscus. The volumetric excess of the monomer in respect to the mold cavity volume and thus also the height of the meniscus affect the optical power of the lens. For this reason, it is possible to create lenses of various optical power using one mold, by metering various volumes of liquid monomer mixture into the cavity.
A typical product of this process is a biconvex intraocular lens with a diameter over 9 mm, central thickness 2.5 to 6.3 mm, frontal optical surface in the shape of a flat ellipsoid with a center radius from 7.5 to 15 mm, rotationally symmetrical rear optical surface with a center radius from 5 to 8 mm and toric transitional zone between both optical surfaces.
This typical product has several disadvantages. First, the central thickness and the overall lens volume are too high for a small-incision implantation, which is required by today's surgeons. Secondly, the front optical surface in the shape of a flat ellipsoid is not advantageous, because it has a high spherical aberration. Furthermore, this surface is often unevenly deformed by the polymerization in various parts of the monomer mixture proceeded at various rates, therefore the contraction was not entirely uniform. This is a general disadvantage of static polymerization casting in open molds.
The U.S. Pat. No. 4,846,832 describes a soft biconvex intraocular lens with a rear convex surface formed by the solidification of liquid meniscus, while the front surface has a central convex zone with a diameter of 4 to 8 mm. The central zone is surrounded by the concave surface of a relatively thin peripheral zone. The front side of the lens is therefore created by the imprint of the mold with a concave optical center zone and a sharp circular rim at the edge. Since the rear side is formed by a solidified meniscus of the liquid, it probably also has the shape of a flat ellipsoid and therefore also has a high spherical aberration. Intraocular lenses can be manufactured using this method either using silicone rubber, or a cross-linked hydrogel with a relatively high index of refraction at 1.42 to 1.43.
The U.S. Pat. No. 5,674,283 describes an intraocular lens in the shape of a saucer with a biconvex optical zone, similar to the product of the preceding method, but made using a different method. The main difference is that in case of a lens made according to the U.S. Pat. No. 5,674,283 the rear side of the lens is shaped by the cavity of the mold, while the front side of the lens is shaped by the meniscus of the monomer, i.e. like in case of a lens made using the method in U.S. Pat. No. 4,971,732, and conversely of the lens made according to the method described in the U.S. Pat. No. 4,846,832. This method is a modification of the method according to the U.S. Pat. No. 4,846,832 with the difference that the optical surface creation by the use of the meniscus is used only in the central area of the lens. This method uses a two part form, where the top part of the mold has a central circular window, in which the meniscus of the liquid monomer is formed. The diameter of the optical zone is substantially smaller than the diameter of the whole lens.
The goal of the invention is to improve the production of intraocular lenses, which has been until now done using stationary casting in open molds, described in the U.S. Pat. Nos. 4,971,732, 4,846,832 and 5,674,283, and thus obtain a wider selection of attainable shapes and optical power values, improvement of optical quality, and increase of production yield of intraocular lenses. In the framework of the invention, the technical prejudice according to which centrifugal casting was deemed unusable for the production of intraocular lenses by the technical community, has been overcome, and with a surprise the reverse has been found, that under the conditions defined by the invention it was possible to produce intraocular lenses of required quality using centrifugal casting. Even though the centrifugal casting, used within the framework of the invention, was originally developed for contact lenses, the conditions for the production of intraocular lenses according to the invention and the hitherto used conditions for the production of contact lenses are entirely different and not deducible from one another, because the conditions, which are suitable for intraocular lenses, are not usable for contact lenses, and vice versa. The method according to the invention differs from the current state of technology even by technical problems, which it solves. While in case of implantable intraocular lenses the invention solves problems of stationary casting in open molds and the removal of some of its' shortcoming, the spin casting of contact lenses did not originate in stationary casting in open molds, because contact lenses cannot be made by stationary casting in open molds at all, and casting while rotating in case of contact lenses thus did not attempt to solve technical problems of stationary casting, but it solved entirely different set of technical problems, which are problems associated with contraction in closed molds. In the scope of this invention it was discovered, that implantable intraocular lenses can be advantageously manufactured by rotational casting under conditions specified below, while the result of the method according to the invention is, furthermore, an implantable intraocular lens with a unique configuration and useful characteristics.